Method of cleaning sugar crystals

ABSTRACT

A METHOD OF TREATING A MAGMA OF SUGAR CRYSTALS IN SYRUP TO REMOVE IMPURITIES FROM THE CRYSTAL SURFACES IN WHICH THE CRYSTALS ARE SJUBECTED TO SCRUBBING IN THE MAGMA IN THE BOWL OF A CONTINOUS CENTRIFUGE, AND ARE THEN DRAINED AND FED OVER A ROTATING SCREEN ON WHICH THEY MAY BE WASHED

April 20, 1971 I F. x. FERNEY METHOD OF CLEANING SUGAR CRYSTALS Filed Aug. 20, 1968 ww mm .8 MN 9 mm g mm mwDmEkzmo mmjwzzz POI 63m Q25 0E 5E 2922;

United States Patent 3,575,709 METHOD OF CLEANING SUGAR CRYSTALS Francis X. Ferney, Walpole, Mass., assignor to Bird Machine Company, Walpole, Mass. Filed Aug. 20, 1968, Ser. No. 753,966 Int. Cl. B04]: 5/00; C1315 1/10 US. Cl. 12756 8 Claims ABSTRACT OF THE DISCLOSURE A method of treating a magma of sugar crystals in syrup to remove impurities from the crystal surfaces in which the crystals are subjected to scrubbing in the magma in the bowl of a continuous centrifuge, and are then drained and fed over a rotating screen on which they may be washed.

This invention relates to a method of treating a magma of sugar crystals.

In various stages of the processing of sugar, it is required to treat a magma of sugar crystals in syrup so as to remove surface impurities of the crystals, drain the syrup from the mass of crystals, and wash the syrup from the crystal surfaces. Previous methods of treatment typically provide a scrubbing action, capable of removing surface impurities, for only a limited portions of the treatment; provide inefficient drainage; and require such a large proportion of liquid in order to wash the syrup from the crystals that an excessive amount of sugar crystal is dissolved during the wash, thus necessitating expensive recovery cycles.

Accordingly, it is an object of the invention to provide a method of treating a magma of sugar crystals which provides an improved cleaning and scrubbing action on the crystal surfaces while resulting in a reduced amount of crystal dissolving.

The invention features a method for treating a magma of sugar crystals wherein the magma is continuously fed to a rotary impervious centrifuge bowl to form therein a pool in which most of the sugar crystals of the magma separate from the liquid under centrifugal force to form a bed of the crystals held against the inner surface of the bowl by the centrifugal force. Liquid is drained from the pool at a rate to maintain the pool volume substantially constant and the crystals in the bed are scrubbed by pushing and tumbling them along the surface of the bowl and against one another in the pool for sufficient time to remove a substantial amount of impurities therefrom into V the liquid of the pool. The crystal bed is conveyed out of the pool onto an axially sloped drainage section of the bowl and the crystals are further scrubbed by pushing and tumbling the same along the surface of the drainage slope while draining a substantial part of the liquid from the bed. The crystal bed is conveyed from the smaller radius end of the drainage slope to the inlet end of a rotating screen and further scrubbed by pushing and tumbling the crystals over the inner surface of the screen drum to the outlet end thereof while draining further liquid from the bed through the screen drum.

Further features include such a method wherein a wash is applied to the crystal bed during its residence in the screen drum; wherein the Wash is Water of the order of 1% by weight of the crystal bed in the screen drum; and wherein the crystal bed has a residence time in the pool of at least 8 seconds, a residence time on the drainage section sufficient to reduce the surface liquor content to about or less, and a residence time in the screen drum of at least 9 seconds.

Other objects, features, and advantages will appear from the following description of a preferred embodiment of 3,575,709 Patented Apr. 20, 1971 the invention, taken together with the attached drawings in which:

FIG. 1 is a schematic illustration of a typical afiination process in the refining of cane sugar; and

FIG. 2 is a longitudinal section of a centrifuge as used in the method of the invention.

The stages in the processing of sugar wherein the method of treatment according to the invention may be advantageously employed to clean the crystals and separate them from the syrup are as follows.

RAW CANE SUGAR After the steps of crushing, clarification, evaporation, and crystallization, a magma of sugar crystals in a syrup is derived which must be processed to clean the crystals and separate them from the syrup.

REFINED CANE SUGAR The afiination process is a suitable process for the application of the method of treatment of the invention. It comprises crystallization, hot mingling, and a processing of the resulting magma to wash the sugar crystals and separate them from the syrup.

BEET SUGAR In the late stages of the refining of beet sugar, a magma of sugar crystals in syrup is derived. The washing of the crystals and their separation from the syrup is then required.

For purposes of illustration, the invention will be particularly described as utilized in the afiination process in the refining of cane sugar, it being understood that this is representative of the other uses mentioned above.

As shown in FIG. 1, in the aflination process raw sugar and affination syrup are transmitted to a hot mingler in approximate equal proportions, the syrup being pre-heated to approximately F. The tumbling of sugar crystals against one another in the hot mingler produces a scrubbing action which tends to remove solid and liquid impurities from the crystal surfaces. These removed impurities are dissolved or suspended in the aiiination syrup which carries them away as effluent from the ensuing treatment.

After hot mingling, separation of sugar crystals from the affination syrup takes place. In accordance with this invention, during this step, continued tumbling of the sugar crystals under centrifugal pressure is maintained to extend the scrubbing action. Under continued tumbling, there follow steps of draining residual free-flowing syrup from the mass of crystals and removing syrup from the crystal surfaces.

It has been discovered that a commercially available centrifuge, when certain operating parameters are maintained, can successfully perform all steps subsequent to hot mingling as described above. FIG. 2 is a longitudinal section of such a centrifuge.

The centrifuge shown in FIG. 2 comprises a base 10 carrying at one end a bracket arm 12 supporting a fixed feed pipe 14 through the inlet end 16 of which is introduced the magma. The centrifuge 'bowl indicated generally by the numeral 18 is mounted for rotation about a central axis. Bowl 18 comprises imperforate cylindrical portion 20, imperforate frustoconical portion 22, and cylindrical screen portion 24.

Mounted concentrically within bowl 18 is a conveyor 26 mounted for rotation and having helical flights 28 secured to a hollow cylindrical core 30. In the preferred embodiment the conveyor 26 is a double helix having a live inch lead length. Inlet pipe 14 extends into the interior of hollow core 30 which is provided with apertures 32 to permit the feed magma to pass from feed pipe 14 into the interior of bowl 18. A pipe 34 for Wash fluid extends concentrically through the center of feed pipe 14 and discharges into a compartment in core 30 provided with nozzle outlets.

Bowl 18 and conveyor 26 are driven by drive mechanism (indicated at the left hand end of FIG. 2) in the same direction but at different speeds so that conveyor 26 rotates slowly with respect to bowl 18. This arrangement causes the conveyor to advance the layer of solid particles deposited upon the inner surface of the bowl from the inlet end of the bowl toward the left hand or solids discharge end as seen in FIG. 2.

A casing 36 is mounted around the outside of bowl 18 and is provided with a solid discharge opening 38 and a liquid discharge opening 40. A customary baflle system serves to maintain the solids discharge separate from the liquid discharge and the liquid discharges from the imperforate and screen sections separate. Specifically annular baffles 46, 48, 50 fixed to the inside of casing 36 mate with corresponding baffles 46, 48, 50 secured to the outer face of bowl 18 for thi purpose. If desired an additional baflle 52 can be provided inside the casing 36 along with outlets 54 and 56 to segregate the liquid dis charges (wash liquid or mother liquor) from the various parts of the screen portion.

As the magma is introduced into bowl 18 it is formed into a pool by centrifugal force against the imperforate inner surface. The surface of the pool intercepts the conical surface of the bowl at a zone A, the location of which is determined by the position of liquid overflow port 42.

If desired, a system of spray nozzles 58 connected to a suitable header or manifold 60 may be mounted on the part of casing 36 radially outwardly of screen portion 24. By supplying water or steam to the manifold when desired the nozzles may be used to clean the screen portion 24.

In operation of the device the bowl 18 and conveyor 26 are driven at appropriate differential speeds while the magma is introduced through inlet opening 16 and flows through inlet pipe 14 thence through apertures 32 in core member 30 to form a liquid pool by centrifugal force against the inner surface of the imperforate conical portion of bowl 18. As the level' of liquid within the pool rises above the margin of liquid overflow port 42, the liquid overflows into casing 36 and thence through a liquid discharge opening 40 in the bottom of the casing. The solid particles in the magma, being of greater density than the liquid, form a layer directly against the inner surface of bowl 18 and are urged along it by flights 28 of conveyor 26 from right to left as seen in FIG. 2. As the layer of solid particles emerges from the liquid pool and is further advanced along the conical inner surface of the bowl toward the inlet end of screen portion 24, liquid continues to drain by centrifugal force and to flow back toward the liquid pool. This initial drainage of a large proportion of the liquid from the solid particles changes the flow characteristics of the mass of solid particles so that as it continues to be advanced by conveyor 26 across the screen portion 24 of the bowl, tendency of the apertures in the screen to become plugged or blinded by the solid particles is minimized, and additional and greater drainage of the liquid from the solid particles is achieved. Wash liquid or vapor (e.g. water or steam) may be introduced through pipe 34 to wash and/or dry the solid particles passing across screen portion 24. The mass of solid particles having been effectively washed and separated from the liquid portion is discharged through apertures 44 into casing 36, from which it passes through a solids discharge outlet 38. The action of helical conveyor 26 assures a tumbling and scrubbing action for the entire residence time of the sugar crystals within bowl 18.

The invention has been successfully practiced with a centrifuge as just described wherein the bowl 18 was 28 inches in length and had a diameter, at its widest point, of 18 inches. Bowl 18 was rotated at a speed of about 2000 rpm, developing a centrifugal force of about one thousand times gravity at the l8 inch diameter. The feed rate and rate of rotation of the conveyor relative to the bowl were such as to provide a residence time of the crystals in cylindrical portion 20 of bowl 18 above about 8 seconds and in cylindrical screen portion 24 above about 9 seconds, the preferred minimum residence times in the practice of the process. The drainage time between the pool and the screen section was such as to reduce the liquid content of the solids bed to 10% or less, as is desirable. The size and speed of rotation of the bowl can of course be varied substantially while still observing the above preferred residence times provided sufiicient centrifugal force is developed to effect efficient solids separation from the liquid in the imperforate portion and liquid separation from the solids in the screen portion.

Previous methods of cleaning the sugar crystals and separating them from the syrup have suffered from limitations best exemplified by reference to the typical batch centrifuges used therein. Specifically, unlike the present method in which the sugar crystals are caused to rub against each other and against the conveyor helix and the bowl and screen during the separation, draining, and washing steps, in the batch centrifuge the deposited crystals remain stationary relative to each other, and therefore scrubbing is confined to the hot mingling step. Also, as mentioned above, the batch machines are best used, as is well-known, with a feed magma which is low in liquid content. Mingling has been done, therefore, with relatively small quantities of syrup, thereby reducing the effectiveness of the mingling step. The centrifuge chosen to implement the method according to the invention entirely circumvents these difficulties and, in fact, operates best with thinner magma. In addition, the typical 5 to 7 inch thick crystal cake in a batch centrifuge is most difficult to wash, due both to the thickness and the stationary aspect of the crystals in the bed (which results in channeling of wash water through the cake). When operating with the parameters as above described, the centrifuge as used herein attained results superior to those of a batch machine (in terms of resultant crystal purity and color) using only one third the wash water typically required with the batch machine (1% vs. 3% of the weight of the crystals to be washed) while avoiding the control complications and processing time loss inherent in loading and unloading the batch machines. Important factors in these improved results are more effective scrubbing, thinner cake, the fact that the tumbling crystals are all exposed to wash water while traversing the screen portion of the centrifuge bowl and less sugar dissolved by the wash water. This decrease in amount of wash water reduces substantially the expense of recovery cycles. Even when no wash was used, results equal to those of the batch machine with usual wash were obtained, showing the effectiveness of the scrubbing produced in the centrifuge.

What is claimed is:

1. Method for treating a magma of sugar crystals which comprises:

feeding said magma continuously to a rotary impervious centrifuge bowl to form therein a pool in which all or most of the sugar crystals of the magma separate from the liquid under centrifugal force to form a bed of the crystals held against the inner surface of the bowl by the centrifugal force;

draining liquid from said pool at a rate to maintain the pool volume substantially constant; scrubbing the crystals in said bed by pushing and tumbling the same along the surface of said bowl in said pool for sufiicient time to release a substantial amount of impurities therefrom into the liquid of said pool;

conveying said crystal bed out of said pool onto an axially sloped drainage section of said bowl and further scrubbing said crystals by pushing and tumbling the same along the surface of said drainage slope while draining a substantial part of the liquid from said bed back into the pool; and

conveying said crystal bed from the smaller radius end of said drainage slope to the inlet end of a rotating screen drum and further scrubbing said crystals by pushing and tumbling the same over the inner surface of said screen drum to the outlet end thereof while draining further liquid from said bed through said screen drum.

2. The method of claim 1 wherein a wash is applied to said crystal bed during its residence in said screen drum.

3. The method of claim 2 wherein said wash is water of the order of 1% of weight of said crystal bed in said screen drum.

4. The method of claim 1 including a pre-mingling of sugar crystals and a sugar syrup to produce said magma.

5. The method of claim 4 wherein said magma is of the order of 40 to 50 brix in sugar content.

6. The method of claim 1 wherein said crystal bed has a residence time in said pool of at least 8 seconds.

7. The method of claim 6 wherein said crystal bed has a residence time on said drainage section sufficient to reduce the liquid content thereof to a maximum of about 10% by weight.

8. The method of claim 7 wherein said crystal bed has a residence time in said screen drum of at least 9 seconds.

References Cited UNITED STATES PATENTS 3,348,767 10/1967 Ferney 2332X 2,464,440 3/1949 Delius 1'27-56UX 3,290,172 12/1966 Dietzel 12756X 3,301,708 1/1967 Rotel 12756X US. Cl. X.R. 12719;2337 

